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Giant anomalous Nernst effect and quantum-critical scaling in a ferromagnetic semimetal

Abstract

In conducting ferromagnets, an anomalous Nernst effect—the generation of an electric voltage perpendicular to both the magnetization and an applied temperature gradient—can be driven by the nontrivial geometric structure, or Berry curvature, of the wavefunction of the electrons1,2. Here, we report the observation of a giant anomalous Nernst effect at room temperature in the full-Heusler ferromagnet Co2MnGa, an order of magnitude larger than the previous maximum value reported for a magnetic conductor3,4. Our numerical and analytical calculations indicate that the proximity to a quantum Lifshitz transition between type-I and type-II magnetic Weyl fermions5,6,7 is responsible for the observed –Tlog(T) behaviour, with T denoting the temperature, and the enhanced value of the transverse thermoelectric conductivity. The temperature dependence of the thermoelectric response in experiments and numerical calculations can be understood in terms of a quantum critical-scaling function predicted by the low-energy effective theory over more than a decade of temperatures. Moreover, the observation of an unsaturated positive longitudinal magnetoconductance, or chiral anomaly8,9,10, also provides evidence for the existence of Weyl fermions11,12 in Co2MnGa.

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Fig. 1: Crystal structure, theoretical band structure and Weyl points of Co2MnGa.
Fig. 2: Observation of the giant anomalous Nernst effect at room temperature in Co2MnGa.
Fig. 3: Giant anomalous Hall and transverse thermoelectric conductivities and the crossover between the regimes following and violating the Mott relation.
Fig. 4: Evidence for the Weyl metal state in the ferromagnetic Co2MnGa.

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Acknowledgements

This work was supported by CREST (JPMJCR15Q5) by Japan Science and Technology Agency, by Grants-in-Aid for Scientific Research (grant numbers 16H02209, 25707030), by Grants-in-Aid for Scientific Research on Innovative Areas “J-Physics” (grant numbers 15H05882 and 15H05883) and Program for Advancing Strategic International Networks to Accelerate the Circulation of Talented Researchers (grant number R2604) from the Japanese Society for the Promotion of Science. P.G. was supported by JQI-NSF-PFC and LPS-MPO-CMTC (at the University of Maryland) and start-up funds from the Northwestern University. The use of the facilities of the Materials Design and Characterization Laboratory at the Institute for Solid State Physics is appreciated.

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S.N. conceived and planned the experimental project. A.N., R.S. and S.N. worked on the single-crystal growth and preparation of samples. A.S. and R.S. carried out the transport and low-temperature measurements and analysed the data. Y.M., T.K., M.S., N.T. and R.A. performed the first-principles calculations. P.G. formulated the quantum critical theory and scaling analysis of the experimental and numerical results. S.N. performed the scaling analysis. R.I. performed the chemical analyses. D.H. acquired the electron diffraction image. S.N., A.S. and P.G. wrote the paper with inputs from Y.M. and R.A. All authors discussed the results and commented on the manuscript.

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Correspondence to Satoru Nakatsuji.

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Supplementary Figures S1–S7, Supplementary Table S1

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Sakai, A., Mizuta, Y.P., Nugroho, A.A. et al. Giant anomalous Nernst effect and quantum-critical scaling in a ferromagnetic semimetal. Nature Phys 14, 1119–1124 (2018). https://doi.org/10.1038/s41567-018-0225-6

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